lumia 520 wont connect to wifi after black update

hello everyone, hope someone can help with problem I have, nokia lumia was working brilliantly with amber update but after black update the wifi loses connectivity intermittently and now a month later simply wont connect to internet. I have reset the phone both soft and hard several times reset router several times went nokia suite and downloaded software, even sent it back to nokia a couple of times. all to no avail. they want me to send it back again which I will do but I thought I might try on here first. is there a wifi patch maybe I could download or would downloading the developer 8.1 update maybe make a difference maybe the firmware would be better on it? I know my warranty would be invalidated if I did that but maybe if I have no other option. anyway its disappointing because I loved the phone before, anyway if anyone can help it would be greatly appreciated, thanks marc